The problem comes when people say, I'm done with crypto. I want U.S. dollars that I can actually spend. When enough people do that, you end up with a "run-on-the-bank scenario". There isn't enough actual cash to exchange for the cryptocurrencies. When word of that leaks, more and more people try to cash out. Then, the whole system implodes like a house of cards.
Most people don't even understand that the banking system works this way. A bank branch may only have enough cash in the vault on any given day to cash-out 5% or 10% of all funds on deposit with that branch. They specifically carry more cash in the vaults on days that are normal paydays, like the 15th and last day of each month, for example, because more people cash their paychecks or even make additional withdrawals on those days. Thee proliferation of debit and credit cards over the last twenty years will have relieved so many of the cash transactions that it wouldn't surprise me if banks hold well below 1% of funds in cash on some days.